Add automated test suite for the Flutter frontend

Introduce a headless `flutter test` suite (83 tests) covering models,
utilities, every API endpoint, and the five screens, with shared helpers
and fixtures so new tests stay terse.

API endpoint methods are statically-dispatched extensions on the
BackendAPI singleton, so they cannot be mocked via `implements`. Mock at
the Dio HTTP-adapter layer (http_mock_adapter) instead, enabled by two
small @visibleForTesting seams: BackendAPI.dioForTesting swaps the
singleton's Dio, and BackendReachability.forceOnlineForTesting() forces a
deterministic online state so polling widgets load.

Add frontend/run_tests.sh and document it in CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/api/backend_api_test_endpoint_test.dart

@@ -0,0 +1,18 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/utils/oott_api.dart';
+
+void main() {
+  // BackendAPI.test() builds its own Dio internally (no injectable seam), so
+  // only its error-mapping path is testable here: an unresolvable host must
+  // come back as a non-null, user-facing message rather than throwing. The
+  // success path is a documented coverage gap.
+  test('BackendAPI.test returns a user message for an unreachable backend',
+      () async {
+    // Loopback port 1 is closed: connection is refused immediately, keeping the
+    // test hermetic (no DNS, no TLS, no traffic leaving the machine).
+    final result = await BackendAPI.test('http://127.0.0.1:1/api', '');
+
+    expect(result, isNotNull);
+    expect(result, isNotEmpty);
+  });
+}